Automatic frequency control system



June 5, 1945. M. e. CROSBY 24,377,326

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AUTOMATIC FREQUENCY CONTROL SYSTEM Filed April 6, 1942 3 Sheets-Sheet 3 a Q g Q l i 4 c mg 8 INVENTQR 4 4- n m Q'\ i a a 63 BY v; m w) M :20 TTORNEY j Patented June 5, 1945 AUTOMATIC FREQUENCY CONTROL SYSTEM Murray G. Crosby, Riverliead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 6, 1942, Serlal No. 437,757

Claims. (Cl. 179-1715) This application concerns a detecting system for automatic frequency control potentials in which the stability of the controlled oscillator is made independent of the usual tuned discriminator circuit. The controlled oscillator output and the reference oscillator output are combined and their resultant is fed to a frequency modulation detector and an amplitude modulation detector. The detected outputs of these two detectors are combined in a balanced modulator detecting system. The phase of the beat note from the amplitude modulation detector reverses as the two oscillator outputs pass through uro-beat synchronism, but the phase of the beat note fro'm'the frequency-modulation detector does not reverse. The result is a detected voltage suitable for AFC potentials since on one side of zero beat the two detected voltages add, and on the other side they subtract. Balanced modulator detection causes this summation to produce a direct current voltage which is zero at synchronism and rises in a positive or negative direction as the controlled frequency deviates either side of synchronism with the reference frequency. The extent of rise in each direction is proportional to the beat note frequency.

A system similar to the system disclosed herein is shown in Hansell U. S. Patent #2,104,801, dated January 11, 1938. In the arrangement of this Hansell patent, however, the detecting system utilizes a phase detection whereas my present invention makes use of frequency modulation detection. There is a parallel difference between the image-eliminating systems of Hans ll U. S. Patent #1044345" my Patent #2,150,l18. Crosby application #393,339, filed May 14, 1941, also discloses a-similar system.

At present, the most'important use for this type of detecting system appears to be in controlled wave length' modulators as disclosed in my U. 8. application #136,578, filed April 13, 1937, now U. S. Patent No. 2,279,659, issued April 14,

1942. With automatic frequency control systems as disclosed in said application, the frequency of the resulting controlled oscillation is dependent upon the tuning of the discriminator so that if there is a change in the magnitudes of the elements of the discriminator, such as might be brought about by temperature or humidity changes, the frequency of th controlled oscillatorvaries. In order to overcome this, temperature-controlled disscriminators and compensating circuits have been resorted to. In the system of the present invention, the need for such expedients, which compensate for temperature, humidity and other effects on the discriminator, is removed since the developed automatic frequency control potentials depend only on the degree of synchronism between the reference oscillator and the controlled oscillator.

In describing my invention, reference will be made to the attached drawings wherein Fig. 1 illustrates schematically by block diagram a wave length modulation system including my novel means for deriving a controlling potential which is independent of or unaffected by changes in dimensions or electrical characteristics of the discriminator circuits used to derive the potential;

Fig. 2 shows an embodiment of one of my novel means for detecting the frequency modulation and ampliture modulation components on wave energy produced by beating together oscillations from a constant or reference frequency source and oscillations of variable or changing frequency from the controllable source;

Figs. 3a to 31:. inclusive are vector diagrams illustrating the manner in which the phase and amplitude of a beat note resulting from the combination of oscillations of fixed frequencies and oscillations of variable frequency varies;

Fig. 2a is a simplified modification of the arrangement of Fig. 2;

Fig. 4 illustrates somewhat completely the essential elements of a wave length modulator and wave transmitter of the stabilized type comprising a controllable oscillator, reactance tubes for controlling the same and a means such as illustrated in Figs. 1, 2 and 2a for deriving controlling potentials for stabilizing the frequency of the controllable oscillator; while,

Figs. 5a to 5c inclusive illustrate the manner in which the outputs of the frequency and amplitude detectors of Figs. 1, 2, 2a and 4 combine to provide a potential which varies in accordance with deviations in the wave length of the controlled oscillator, which potentials are free of the effects of changes in dimensions or other electrical characteristics of the circuits used in deriving the same.

In Fig. l, O is a source of oscillations of constant frequency, such as a crystal controlled oscillator. This is the reference source referred to herein. W0 is a wave length modulated wave generator controlled by modulating potentials and AFC potentials. This is th controllable oscillator. These oscillations are brought together in a mixer M and fed to a frequency modulation detector in D and an amplitude modulation detector in A. The resulting beat notes are supplied to a balanced modulator E including detectors. The resulting potential is fed back to W0 to stabilize the mean wave length of the generated oscillations. F designates the output stages of the system.

The vector diagrams of Figs. 3a to 3h, which are the same as the corresponding figures in my U. S. Patent #2,150,1l5, dated March 7, 1939, show how the reference oscillator voltage El and the controlled oscillator voltage E2 combine to form the resultant ER, which is both phase and amplitude modulated. The different representations 3a, 3b, 30, etc, show successive instances of time. The counter-clockwise direction of rotation is taken as the positive direction. Hence, as E2 rotates counter-clockwise about El, E2 must be higher in frequency than E I By viewing the diagram in the sequence la, 3b, 3c, etc.

112 appears to be the higher frequency and by viewing in the sequence 30, 2h, 8a, 2!, 20, Id, Ic, lb, E2 appears to be the lower frequency.

Fromthediagramsofl ig.3,itcanbeseen that the resultant of the two voltages is modulated in amplitude between the limits of amplitude given by the resultant vectors ER in diagrams 2c and la. It is modulated in phase between the limits given by the phase positions of the resultant vectors in diagrams to and 8e. Careful examination of the vector diagrams in the sequence is, 3h. 3g, 3!, etc., and 80, lb, 3c, etc., will reveal that the phase of the amplitude envelope is 180 apart for the two sequences. However, the phase deviations are in phase for the two sequences. Thus, in the case of amplitude variation, when the controlled oscillator is higher in frequency than the reference oscillator, the envelope has a phase which is opposite to that when the controlled oscillator is lower. In the case of the phase variation, the phase of the variation of the resultant vector does not change on either side of zero beat.

Diagrams 3c and 30 also show that at the time the amplitude envelope is at its maximum or minimum, the phase deviation is going through zero. This indicates a 90 phase difference between the amplitude envelope and what might be called the phase envelope." Thus, if the phase modulation were detected on a phase modulation receiver instead of the frequency modulation receiver used in this invention, a phase shifter would be required in one of the detector output circuits so as to cause the two detected voltages to be exactly in or out of phase. This phase modulation detection is the type of detection employed in the above mentioned Hansell Patent #2,104,80l. In the present invention, a frequency modulation detection is used so that the phase shift inherent to the detection of phase modulation on a frequency modulation detector is utilized and no separated phase shifter is necessary. Frequency modulation is the rate of change of phase modulation. The rate of phase change is 90' displaced from the actual value of phase, so that when the phase deviation is maximum, the frequency deviation is minimum.

The circuit of Fig. 2 shows the essential elements of the detector system shown at M, A, D, E, and E of Fig. 1. The input, consisting of the combination of the voltage supplied by the reference oscillator O and that from the controlled oscillator W is fed to the grid II and ground and cathode l2 connection of tube ii. The anode ii of this tube feeds output to a frequency modulation discriminator circuit including tuned circuits 2| and 2| which are off tuned to opposite sides of the frequency of the oscillations supplied by the reference oscillator, or a harmonic of the reference oscillator frequency if frequency multiplication is used before the combined oscillations are fed to tube It and thereby applied to the frequency discriminators. These oscillations are modulated in phase and in amplitude by the deviations in wave length of the controlled oscillations. Detectors Ill and 30' make the frequency modulation available at the point marked FM and the amplitude modulation component at the point marked AM.

The detectors here operate as disclosed in my United States application #401,784, filed July 17, 1941, now U. 8. Patent No. 2,296,092, imued September 15, 1942. Tuned circuits 2. and 2| are oi! tuned to either side of the carrier so that the amplitude modulation converted from the frequency modulation and fed to detector II is of opposite phase to that fed to detector 30'. The two detectors are arranged in a series connection so that the portion of the detected voltage from 2., appearing on resistor 2|, is added to the corresponding voltage from 8| which appears on 2|. 'Ihese voltages are added in push-pull, or with a 180 phase reversal between them since the anode side of resistor 2! is the low potential side while the anode side of resistor 2| is the high potential side. Consequently the detected frequency modulations will be added in proper phase to form an aiding combination. Resistors 22 and 22' are also detector resistors which have a portion of the detected voltage appearing across them. However, since these latter resistors are in the cathode circuits, their detected voltages will be of opposite phase to that across the resistors in the plate circuits (for instance, the voltage across the cathode resistor 22 will be of opposite phase to that across plate resistor 2|). The values of the cathode and plate resistors are normally made equal so that equal portions of the detected voltages appear across them. This reversal of phase effected by switching to the cathode resistor allows the in-phase combination of the detected voltages from detectors II and II. This in-phase combination produces the amplitude modulation component of the incoming signal. The amplitude modulation component appears at point All as g :gmbination of the voltages on resistors 2| an Resistance filters 34 and II remove the radiofrequency component from the detected output The frequency modulation is fed to grid ll of tube Ii and the amplitude modulation is fed to grid 42 of tube ll. Tube Cl and ll amplify the detected frequency modulation and amplitude modulation outputs, respectively. The frequency modulation is fed to the push-pull input of the balanced modulator detectors '2 and 54 by transformer l0 and the amplitude modulation to a winding of the push-push input transformer 82. The automatic frequency control potentials appear across the diode resistors II and 5'' which are connected in series opposing so that a differential detection is obtained.

The output of the frequency modulation detector at point "PM" will consist of a beat note which is effected by the combination of the reference oscillator and the controlled oscillator. This beat note does not change in phase as the two oscillators go through zero beat, but since the detection is one of phase modulation on a frequency modulation receiver, the amplitude characteristic rises with frequency as the controlled oscillator departs further away from synchronism. Thus, as the beat note is varied between one side of zero beat and the other. the amplitude of the output of the frequency modulator detector drops from a maximum at one side of zero beat to zero at zero-beat frequency and then up to maximum again at the other side of zero beat Such an output is shown in Fig. 5a, curve B.

The output of the amplitude modulation detector will be constant in amplitude as shown in Fig. 5a, curve A but as the frequency goes through zero beat the polarity or phase of the detected output will reverse.

When these two detected outputs are fed to the balanced modulator with the two inputtransformers 50 and 53 which combine one pair of voltages in phase and the other pair of voltages out of phase, at one side of zero beat, one detector, say 52, will receive the sum of the two voltages and the other detector, say 54, the difference of the two voltages. This will cause one detector output to predominate over the other so that the resulting detected output is predominately positive or negative as the case may be. At zero beat, both detectors will have the same voltage applied to them so that the difference of their detected outputs, which appear on the series-opposing connected resistors 55 and 56' is balanced giving zero voltage. The result of this type of combination is the overall detection characteristic as shown in Fig. 5c. This is the desired characteristic for automatic frequency control potentials.

Fig. 5a shows curves of the amplitude characteristics of the detected amplitude modulation, the detected frequency modulation, and'the combination of these two detected outputs. Curve A is the flat output which is obtained from the amplitude modulation detector. This type of beat note output would be obtained from the detected amplitude modulation component of the combination of the controlled oscillator and the stable or reference oscillator. F is the reference oscillator frequency and the frequency scale represents the variation of the controlled oscillator frequency.

Fig. b shows the phase characteristic of the amplitude modulation beat note output. As the controlled oscillator goes through synchronism with the reference oscillator frequency, the phase shifts a total of 1r radians or 186.

Curve B of Fig. 5a is the characteristic of the detected output of the phase modulation component of the combination of the two oscillators. Since the detection ofthe phase modulation component is on a frequency modulation receiver, the output is proportional to the frequency of the beat note so that it drops to zero as the two frequencies Pas through synchronism. There is no change of phase as this component passes through synchronism.

The detected outputs having the amplitude characteristic A, with its phase characteristic of Fig. 5b, and the amplitude modulation characteristic of curve B are combined in the balanced modulator detector. The input to one of the balance modulator detectors is the in-phase combination of these two voltages and to the other the out-of-phase combination. Hence, one detector will be fed the difference between the two characteristics and the other the sum. Thus, at the frequency a, the amplitude of the phase modulation output is equal to the amplitude of the amplitude modulation output so that, for the first of the balanced modulation detectors, the two outputs will cancel to-produce zero output at the point a. At the frequency of the reference oscillator, Fo, there is no output from the frequency modulation detector so that the amplitude is e which is the same as the amplitude of the amplitude modulation component alone. On the other side of F0, the phase of the amplitude modulation detected component reverses. This causes the two components to add so that at the frequency 1 the amplitude is equal to the sum of curves A and B or d.

The input to the second of the balanced modulator detectors is the opposite combination of the two voltages so that addition is obtained on the low-frequency side and subtraction on the high-frequency side. This produces the characteristic of ce-b which is a sloping filter characteristic having a slope opposite to that of curve a--ed. Thus, the two balanced modulator detectors are fed by the well-known crossed-filter characteristics of a frequency-modulation discriminator. The detected output of the balanced-modulator detectors will then have the output characteristic shown by Fig. 5c. The result is, therefore, a potential which is the same as that obtained from a frequency modulation or automatic-frequency-control discriminator and detectors. However, in the case of the present invention; the location of the crossing point of the slope filters, F0, is not dependent upon the tuning elements of the discriminator, but only on the frequency of the reference oscillator.

In Fig. 2a is shown a simplified arrangement in which the audio amplifier 48 and 48 of Fig. 2 for-the detected beat note are dispensed with. In the circuit of Fig. 2, it will be noted that the resulting voltage fed'to the final detectors 52 and It isa combination of the detected frequency modulation output and the detected amplitude modulation output. Such a combination is available dllec'tly from the detector resistor of a balanced frequency modulation discriminator and detector. In Fig. 2a a discriminator and detector of the type shown in Seeley U. S. Patent #2,l21,103 is used. This discriminator in its operation is well known in the prior art and a detailed description thereof is believed unnecessa y at this Point. It will be noted, however, that the anode ll of tube I8 is coupled to a tuned circuit 58, the latter being inductively coupled to a tuned circuit 59 connecting the anode electrodes of diodes CI and in push-pull relation. The circuits l8 and I8 are tuned to the frequency of the reference oscillator. These diodes have output resistors 88 and 10 connected between the oathode with onecathode, that of 64 grounded. The output circuit of tube I6 is also coupled by condenser in phase to the anodes of the diodes 6| and 84. This discriminator circuit linearly converts phase and frequency deviations on the wave energy supplied to the grid l0 into corresponding amplitude deviations on the said wave energy,.which deviations are then rectified in diodes 6i and N. Amplitude variations are also impressed on the diodes 60 and 64 rectified therein and appear across the output resistors 68 and 10.

Due to the opposite slopes of the sloping filters feeding phase frequency and amplitude modulation to each detector one diode resistor, say 68, has the summation of the detected amplitude modulation and the frequency modulation, while the other diode resistor II has the same summation except that the polarity of the frequency modulation is reversed by virtue of the opposite slope of the discriminator. Hence, a combina tion of the detected voltages is available which is the same sort of combination that was effected by the balanced modulator detector including transformer I! and I3, and diodes i2 and 54 of Fig. 1. In the arrangement of Fig. 2a, the final detectors 5! and II are connected to separately detect the outputs of the separate diode resistors 08 and II of the discriminator detectors '80 and 44. These final detector resistors are connected in the series-opposing connection which gives the desired differential detection.

In the above description, it has been assumed that the output of M, Fig. 1 is a carrier modulated in phase and in amplitude in accordance with wave length deviations on the oscillations from W as illustrated in Figs. 3a to 3b. We may, of course, assume that the output from M is a carrier modulated in amplitude and in frequency in accordance with the wave length deviations of WO. This voltage may be represented by a fixed vector about one end of which a vector, of less length, representing the voltage from W0 rotates in a direction which reverses when the two frequencies pass through synchronism. This provides a resultant voltage, the effective frequency of which increases with departure from synchronism and the rotation of which is in a direction depending on the relative frequencies of voltages from O and W0. The resultant carrier is 'also amplitude modulated and the amplitude is maximum or minimum when the frequency is maximum. Then we would have frequency modulation on the frequency modulation detector D and modulation out of D that increases in amplitude as the voltages increase in frequency from synchronism in either direction, and from the amplitude modulation detector A a constant beat note output with phase reversal as synchronism is passed through. These modulation voltages being co-linear add on one side of the balance modulator E and subtract on the other side thereoi.

The direct current output of the balanced detectors of E moves up and down about a mean value, .which is the correct automatic frequency control potential for correct mean carrier frequency from W0.

Note that this operation gives a potential which depends entirely on relative frequencies of the oscillations from O and W0 and is independent of tuning of discriminator circuits of the oif-tuned type Fig. 2 or the Seeley type Fig. 2a

as long as operation takes place on a linear portion of their characteristic even if the said circuits become de-tuned relative to the center frequency of the voltage out of M.

In Fig. 4, I have shown somewhat completely a frequency modulation system in which the automatic-frequency-control means of the present invention is used. In this system, 12 is the main generator, referred to hereinbefore as the controlled oscillator, while 99 is the stable oscillator referred to hereinbefore as the reference oscillator. The oscillation generator 12, which is of the nature described in my above referred to application, has a tank circuit 14 wherein the oscillating energy appears coupled b condenser 16 to the grid ll of amplifier tube It, while the stable oscillator has a tuned circuit 8! coupled by condenser 94 to this grid l0. The oscillator 12 has its tank circuit 14 connected to the anodes of a pair of reactance tubes 98 and 86. The cathodes of these reactance tubes are connected to the cathode of the oscillator 12 by way of ground so that the impedances between the output electrodes of the reactance tubes are in shunt to the plate portion of the tank circuit 14. The control grids of the reactance tubes are connected by phase shifting net works C and R and L and R to the tank circuit 14. The connections are such that one tube acts in a well known manner as a variable capacity included in the tank circuit 14, while the other tube acts as a variable inductance in this tank circuit. The reactance tubes are differentially modulated by potentials applied to a transformer 49 connecting the second grids of the tubes in phase opposition;

This main oscillator and reactance tube arrangement operates to modulatethe oscillations generated and supply them to an output circuit 9|, which may be coupled to the tank circuit 14. As is known, the frequency of the main oscillator may drift and to correct this, I supply oscillations from this main oscillator to the amplifier tube It to beat therein with oscillations from the source 89.

As described in detail above, these oscillations feed energy to a discriminator and detector cir cuit similar to the one of Fig. 2a, so that at the output of the detectors 52 and 54', I derive a potential which varies an extent that corresponds to deviations in the mean frequency of the main generator 12 and of a polarity which corresponds to the direction of deviation of the mean frequency of the generator 12 from its normal frequency. This potential is supplied by time constant circuit SI and line 93 to the controlling electrode of an alternating current phase reversing and amplifying tube 92. This tube 92 has its electrodes 94 and 96 supplied by alternating current through a transformer 99 coupled with an alternating current line I90. The amplitude of one of these potentials, say that on grid 94, is adjustable by adjusting potentiometer 99. The anode I92 of the amplifier and phase reverser tube 92 is connected to the control grid Iii of tube I05 by condenser Hi4 and resistance Hi! to amplify the alternating current output of tube 92 and supply it to one winding H0 of a motor II2. This motor has a second winding IIG connected to the source I00 by lines 129. The currents in the winding H0 and H6 are in phase quadrature. A phasing condenser I22 is included in this connection to winding H6 to establish this quadrature relation. A blocking condenser I24 is included in the connection between the grid 94 and the potentiometer 99 to isolate the winding of transformer 99 from the control potential current in lead 93.

The tube 92 operates as disclosed in my U. S. application, Serial No. 393,339, filed May 14, 1941, to produce in the winding H9 alternating current of a phase determined by the potential supplied by lead 93 to grid 94.

That is as pointed out in said last mentioned application the alternating currents applied to grids 94 and 96 are of like phase. The unbypassed resistance IIH supplies bias for tube 92. The screen grid H3 is charged positive by resistance I I5, while the control grid 99 is biased negative by cathode resistance MI and source I99. In the absence of control potential on lead 93 potentiometer 99 is adjusted so that the voltage amplified by grid 99 just balances out the voltage amplified by grid 94 in the output resistance lil. Under these circumstances no alternating current is supplied to winding H0 and the motor is stationary. If positive voltage is applied from line 93 to grid 94 balance in tube 92 is upset and voltage of a phase amplified by grid 96 is fed to winding H9 and the motor rotor turns in one direction. When less positive potential is fed to grid 96 the voltage amplified by grid 94 predominates and the current in winding III] is reversed in phase because the voltage amplified by grid 94 is opposite to that amplified by grid 96.

The movement of rotor of the motor H2 depends on the relative phases of these currents and the relative phases of these alternating currents depends on the potential on 94. Rotation of the motor rotor H2 tunes the condenser I26 in shunt to the tank circuit 12 and the arrangement is such that this tuning of this tank circuit compensates any tendency of the oscillator frequency to drift.

The generator 80 may include frequency multipliers and the tuned circuits 2!! and 20 of Fig. 2 are then off-tuned with respect to the harmonic output thereof, while the circuits 58 and 59 of Fig. 2a are tuned to said harmonic.

I claim:

1. In a system for producing potentials characteristic of deviations in the frequency of wave energy, a source of oscillations of substantially constant frequency, a circuit, connections for impressing said wave energy and oscillations from said source on said circuit for combining 'said oscillations with said wave energy to produce a resultant wave the amplitude and instantaneous frequency of which is modulated in accordance with deviations in the frequency of said wave energy, an amplifier coupled to said circuit to amplify said resultant wave, an amplitude modulation detector coupled to said amplifier for detecting the amplitude modulation component on said resultant wave, a frequency modulation detector coupled to said amplifier for simultaneously detecting the frequency modulation component of said resultant wave, and a modulation combining and rectifying circuit coupled to said detectors.

2. In a system for producing potentials characterisitic of deviation in the frequency of wave energy, a source of oscillations of a substantially constant frequency substantially equal to the frequency of said wave energy, a frequency discriminating circuit having an input and an output, connections excited by said wave energy coupling said source of oscillations to the input of said discriminating circuit whereby said oscillations and wave energy are combined and impressed on said discriminating circuit and the amplitude and length of the said oscillations are modulated in accordance with deviations in the frequency of the wave energy, the length modulations on said oscillations being converted to corresponding amplitude variation on the oscillations by said discriminating circuit, a demodulation system having an input coupled to said discriminating circuit, said demodulation system comprising a pair of detectors arranged to detect the amplitude variations on said oscillations corresponding to the amplitude modulations and to the length modulations and supp y as output modulation components corresponding to said length modulations and said amplitude modulations of said oscillations, and means for obtaining a potential characteristic of the two demodenergy are combined and the amplitude and the length of the said oscillations are modulated in accordance with deviations of the frequency in the wave energy, the so modulated oscillations being impressed on said two tuned circuits, two diode detectors having their anodes coupled differentially by said tuned circuits and by a pair of impedances and having their cathodes coupled by other impedances, a pair of rectifiers having electrodes coupled in push-pull relation by a first input impedance and in parallel relation by a second input impedance, said rectifiers having electrodes also coupled by output impedances, a coupling between said first input impedance and the impedances coupling the anodes of said detectors, a coupling between the second input impedance and one of the impedances coupling the anodes of said detectors and one of the impedances coupling the cathodes of said detectors, and means for deriving from said output impedances said produced potential characteristic of deviations in the frequency of the wave energy.

4. In a system for producing potentials characteristic of deviations in the frequency of wave energy, a, source of oscillations of a substantially constant frequency substantially equal to the frequency of said wave energy, connections on which said wave energy and oscillations from said source of oscillations are impressed whereby said oscillations and wave energy are combined and the amplitude andllength of the said oscillations are modulated in accordance with deviations in the frequency of the wave energy, a pair of detector tubes each having an anode and a cathode, a circuit tuned to the frequency of said oscillations coupled between the anodes of said tubes, a pair of impedances coupled between the cathodes of said tubes, 9. coupling for applying said modulated oscillations to said tuned circuit, a circuit independent of frequency between said pling and the anodes of said tubes, a pair of rectiflers having electrodes, a coupling between electrodes of one rectifier and one of said impedances, a coupling between electrodes of the other rectifier and the other impedance and an output circuit coupled to said rectiflers.

5. In a wave length modulation system, a controllable wave generator for producing wave energy and modulating the wave length thereof in accordance with signals, said wave generator having a tunable reactance which controls its instantaneous frequency of operation, said generated wave energy being subject to variations w in the mean frequency thereof in addition to ulation components combined in phase or in phase opposition including a pmr of rectiflers having inputs coupled to the output of said demodulation system and having an output wherein said potential appears.

3. In a system for producing potentials characteristic of deviation in the frequency of wave energy, a source of oscillations of a substantially constant frequency substantially equal tothe frequency of said wave energy, two tuned circuits tuned respectively above and below the frequency of said oscillations, connections excited by said tuned circuits whereby said oscillations and wave variation in accordance with signals, a source of oscillations of substantially constant frequency, a frequency discriminating circuit, connections for combining oscillations from said source and modulated generated wave energy from said wave generator, and impressing the same on said frequency discriminating circuit, said combined energy lbeing modulated in amplitude and instantaneous frequency in accordance with deviations in the mean frequency of said wave energy, an amplltude variation detector having an input coupled to said discriminating circuit, said detector having an output wherein potentials characteristic of the frequency deviations and potentials characteristic of the amplitude variation appears, and a frequency controlling circuit coupling the output of said detector system to the tunable-resistance in said oscillation generator to stabilize the mean frequency of operation thereof in accordance with the combined potentials.

MURRAY G. CROSBY. 

